Donor/Acceptor-Copolymers as so-called "low-bandgap" π-conjugated polymers have been widely investigated and applied in different areas such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). Due to the attractive features of those functional organic polymer materials such as light weight, flexibility and produced in a cheap way they have attracted recent interest in research development. Especially so-called "bulk heterojunction"-type solar cells consisting of blends of conjugated donor polymers and "small" acceptor molecules have gained enormous attention to their potential for solar ernergy energy conversion.
In chapter 2, the synthesis of two series of random polyfluorene-type copolymers with on-chain benzo[g]quinoxaline or quinoxaline acceptor units are described. The copolymers have been characterized by gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), absorption and emission spectroscopy. Thermal properties were analyzed by differential scanning calorimetry (DSC). For first OLED investigations single layer devices based on the random copolymers as emissive layer were built. Additionally, by adding a thin semiconducting polymer interlayer [poly(9,9-di-n-octylfluorene-2,7-diyl)-alt- [1,4-phenylene-(4-sec-butylphenylimino)-1,4-phenylene] (TFB)] between the hole transporting (PEDOT:PSS) and the emissive random copolymer layer OLED devices with improved performance have been reported. Using a thin TFB interlayer an enhancement of the luminance efficiency of 172 % was observed. The OLED experiments are presented together with atomic force microscopy (AFM) investigations of the film morphology.
Chapter 3 presents the synthesis and characterization (NMR, GPC, UV/Vis and photoluminescence spectroscopy) of a series of alternating fluorene/dithienyquinoxaline copolymers with a different substitution pattern at the quinoxaline acceptor unit. The thin film properties of the novel materials were investigated by using atomic force microscopy. Single layer polymer light emitting diodes have been prepared by blending 3 wt.-% of the novel alternating copolymers with poly(9,9'-di-n-octylfluorene-co-benzothiadiazole) (F8BT). This blend composition provides optimimum device efficiency.
Chapter 4 introduces the synthesis and characterization of novel alternating quinoxaline/oligothiophene donor-acceptor copolymers. The optical and electronic properties of theses materials showed an unexpected independence of absorption and photoluminescence, as well as of the HOMO/LUMO energy levels, on the length of the oligothiophene segments in the copolymer main chain. The electrochemical properties of the quinoxaline/oligothiophene copolymers were investigated by cyclic voltammetry (CV). For OFET investigations devices based on these copolymers were built up by solution processing (spin-coating). The charge caries mobilities and on/off ratios of thin layers of the quinoxaline/oligothiophene copolymers are increasing slightly with increasing number of thiophene rings of the oligothiophene segments. In this chapter also, first OPV investigations were carried out with the quinoxaline/oligothiophene copolymers. The power conversion efficiencies of the resulting "bulk heterojunction"-type solar cells are reported.
In chapter 5, the synthesis and characterization of further donor-acceptor copolymers based on bisthiazole and thiophene units are described. The solubility of the bisthiazole/thiophene copolymers was very low and, therefore, a solution-based device fabrication was problematic. The bisthiazole/thiophene copolymers have been used as hole transporting material in first OFET investigations.
In the last chapter, the synthesis and characterization of novel cyclopentadithiophene (CPDT)- based alternating copolymers is described. For a successful application in "bulk heterojunction"-type solar cells CPDT-based copolymers with a mean average molecular weight > 10.000 g/mol seems necessary. The novel high molecular weight copolymers showed rather promising power conversion efficiencies of > 2 % in first device experiments. The morphology of blends of the novel donor materials with fullerene acceptors were investigated by AFM. For these copolymers, also OFET measurements have been carried out since in addition to the blend morphology the charge carrier mobility within the donor phase plays an important role for the performance of OPV devices.